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Offset Printing - Multiple Options Make It Sweet

In offset printing, trapping is the process used to compensate for variances in registration that can cause undesirable and unsightly artifacts where areas of two different colors meet. You may never have known or bothered to care about this critical part of printing, and we've made it so you don't really need to. But in case you're curious what all the fuss is about, this page is a brief tutorial on the basics.

In printing, a trap is the tiny overlap required between two different colored elements on a printed page. Trapping is only necessary where different colors touch. If different colors do not touch, there is no need for a trap. This document will attempt to explain the basics of trapping and introduce some terminology used by lithographers.

Overprints vs. Knockouts
Trapping is not necessary when one color OVERPRINTS another color. Overprint means that a color is printed on an area of the paper that was previously printed by another color. For example, a designer might want to put black type in a field of blue. The black ink can overprint the blue ink because black ink just looks blacker when it is printed over another color.

However, problems arise when non-black inks overprint one another because printing inks are transparent. For example, if yellow type were printed over the field of blue, the yellow type would appear green. In order for yellow ink to appear yellow, it must be printed on un-inked (white) paper.

Most graphic and page-layout programs let users set elements to KNOCKOUT. When an element is set to Knockout, all colors/images are removed from beneath the object. For example, if the yellow type were set to knockout in our graphics program, a "hole" in the shape of the type would be "knocked out" of any elements beneath the yellow type. The yellow type could then print onto the un-inked paper.

Registration problems arise when the knockout is the exact size as the element that needs to fit into it. In our example, the paper may shrink or stretch after the blue ink is printed. This makes it nearly impossible to get the yellow type to fit into the knockout. Instead of fitting perfectly, the yellow will be shifted "out of registration" and a sliver of un-inked (white) paper can be seen between the yellow and blue ink.

One way to rectify this "tight registration" problem is to make the yellow type a bit larger than the knockout. Doing so creates a tiny (1/300th of an inch) overlap between the blue and yellow inks. This overlap, known as a TRAP, gives the press operator a margin of error for dealing with registration problems. Making an element larger than its knockout is known as making a SPREAD. Another option is to make the knockout a bit smaller than the element that is to print into it. This technique is known as a CHOKE.

Spreads and chokes were traditionally made using photographic techniques on a lithography camera. These effects can now be made on computers using today's sophisticated layout and graphics programs. In our example, the yellow type is spread by putting a stroke (outline) on the yellow type. The stroke makes the yellow type fatter but doesn't make the knockout any fatter because the stroke is set to overprint.

When trying to decide whether to make a spread or a choke, the lithographer looks at the density of the inks. Typically modifications (chokes or spreads) are made to the lighter colored ink so that changes will be less noticeable.

Suppose, for example, the background is yellow and the type is blue. In this case we still want to modify the yellow element because yellow is the lighter color, and our modifications will be less noticeable if made to the lighter color. The blue type is set to knockout and then the knockout is choked (made just a bit smaller) by adding an yellow stroke that is set to "Overprint".

The preceeding examples of simple trapping situations are presented so that designers can be aware of how their designs effect the production/printing process. Trapping jobs properly is effected by several factors including ink density, coverage, the type of paper, the type of press, etc. We recommend that trapping be left to our desktop publishing specialists.

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